Yes, transmission lines are expensive to build, but they very, very cheap to operate. In the long run it is cheaper to connect different areas together than it is for each area to be totally self-sufficient.
Room-temperature superconductors would be really nice — if they ever arrive — but we don’t need them. Good old copper works just fine. The cost of cable is actually only a small part of the total cost of a transmission line.
Small Modular Reactors — if that is what you are talking about — are an interesting concept. I hope they end up being viable. There are a lot of use cases for small, self-contained nuclear power systems. SMRs crop up on the Nuclear thread a fair bit.
Nothing new is needed for Solar and Wind (i.e. Variable Renewable Energy, VRE) at this point. All problems have tried and tested solutions. The main obstacle is just scaling up production — matching supply with demand. It’s more of a finance and investment problem than an engineering one.